This invention relates to a magnetic field generating apparatus, and in particular to a magnetic field generating apparatus suitable for use with a magnetic resonance imaging system.
According to a magnetic resonance imaging system, an object to be inspected, such as a human body or a human head, is placed in a static magnetic field. A radio frequency pulse is applied to the object while applying magnetic field gradients to the static magnetic field to generate from the object a magnetic resonance signal, which is processed to obtain an image of the object.
In order to carry out the magnetic resonance imaging, it is necessary for the static magnetic field to have a strength in the order of 0.05-2.0 T (tesla) and a magnetic field homogeneity of the order of 10 ppm or less in the measuring spherical region of 30 to 50 cm diameter. The static magnetic field may be generated by a resistive magnet, superconductive magnet or permanent magnet.
A typical permanent magnet used for magnetic resonance imaging is disclosed in Japanese patent application laid-open No. 177903/1987. The disclosed permanent magnet comprises a permanent magnet arrangement for generating a static magnetic field in a measuring space in which an object to be inspected is arranged and a yoke arrangement having its polygonal section arranged around the permanent magnet arrangement for forming magnetic circuits for generation of the static magnetic field together with the permanent magnet arrangement.
However, no attention is given in this construction to the need for adjusting the static magnetic field homogeneity, such as by moving the yoke arrangement.